Hey gaia, many new people encounter this issue (I did). When you send QTUM with the wallet, the wallet has to send the entire amount of a previous transaction (using the Unspent Transaction Output model) and then receives change back on a new "change" address that the wallet supplies (although you can override this default behavior). You could think of it as if you were buying a coffee for 3.0, gave the coffee shop a 10.0 note, and got back 7.0 in change. But the 7.0 change comes back to a new address, the "change" address. The wallet correctly sums the balance for all the addresses it knows about, but the Explorer can only show the balance for one address at a time. If you look at your original transaction on the Explorer, you can see the address used to send the change back. Does this help?